thrust::gather_if

Defined in thrust/gather.h

template<typename DerivedPolicy, typename InputIterator1, typename InputIterator2, typename RandomAccessIterator, typename OutputIterator, typename Predicate> OutputIterator thrust::gather_if(const thrust::detail::execution_policy_base<DerivedPolicy> &exec, InputIterator1 map_first, InputIterator1 map_last, InputIterator2 stencil, RandomAccessIterator input_first, OutputIterator result, Predicate pred)

gather_if conditionally copies elements from a source array into a destination range according to a map. For each input iterator i in the range [map_first, map_last) such that the value of pred(*(stencil + (i - map_first))) is true, the value input_first[*i] is assigned to *(result + (i - map_first)). RandomAccessIterator must permit random access.

The algorithm’s execution is parallelized as determined by exec.

The following code snippet demonstrates how to use gather_if to gather selected values from an input range based on an arbitrary selection function using the thrust::device execution policy for parallelization:

Remark

gather_if is the inverse of scatter_if.

#include <thrust/gather.h>
#include <thrust/device_vector.h>
#include <thrust/execution_policy.h>

struct is_even
{
  __host__ __device__
  bool operator()(const int x)
  {
    return (x % 2) == 0;
  }
};
...

int values[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
thrust::device_vector<int> d_values(values, values + 10);

// we will select an element when our stencil is even
int stencil[10] = {0, 3, 4, 1, 4, 1, 2, 7, 8, 9};
thrust::device_vector<int> d_stencil(stencil, stencil + 10);

// map all even indices into the first half of the range
// and odd indices to the last half of the range
int map[10]   = {0, 2, 4, 6, 8, 1, 3, 5, 7, 9};
thrust::device_vector<int> d_map(map, map + 10);

thrust::device_vector<int> d_output(10, 7);
thrust::gather_if(thrust::device,
                  d_map.begin(), d_map.end(),
                  d_stencil.begin(),
                  d_values.begin(),
                  d_output.begin(),
                  is_even());
// d_output is now {0, 7, 4, 7, 8, 7, 3, 7, 7, 7}

Parameters

exec – The execution policy to use for parallelization.
map_first – Beginning of the range of gather locations.
map_last – End of the range of gather locations.
stencil – Beginning of the range of predicate values.
input_first – Beginning of the source range.
result – Beginning of the destination range.
pred – Predicate to apply to the stencil values.

Template Parameters

DerivedPolicy – The name of the derived execution policy.
InputIterator1 – must be a model of Input Iterator and InputIterator1's value_type must be convertible to RandomAccessIterator's difference_type.
InputIterator2 – must be a model of Input Iterator and InputIterator2's value_type must be convertible to Predicate's argument type.
RandomAccessIterator – must be a model of Random Access iterator and RandomAccessIterator's value_type must be convertible to OutputIterator's value_type.
OutputIterator – must be a model of Output Iterator.
Predicate – must be a model of Predicate.

Pre

The range [map_first, map_last) shall not overlap the range [result, result + (map_last - map_first)).

Pre

The range [stencil, stencil + (map_last - map_first)) shall not overlap the range [result, result + (map_last - map_first)).

Pre

The input data shall not overlap the range [result, result + (map_last - map_first)).